GEORGE RUSSELL HARRISON SPECTROSCOPY LABORATORY

The George Russell Harrison Spectroscopy Laboratory is engaged in
research in the field of modern optics and spectroscopy for the purpose of
furthering fundamental knowledge of atoms and molecules and pursuing advanced
engineering and biomedical applications. Professor Michael S. Feld is Director;
Professor Jeffrey I. Steinfeld and Dr. Ramachandra R. Dasari are Associate
Directors. An Interdepartmental Laboratory, the Spectroscopy Laboratory
encourages participation and collaboration among researchers in various
disciplines of science and engineering. Professors Feld, Steinfeld, Moungi G.
Bawendi, Robert W. Field, Daniel Kleppner, Keith A. Nelson, Stephen J. Lippard,
Jeffrey I. Steinfeld, Toyoichi Tanaka, Steven R. Tannenbaum and Dr. Dasari are
core investigators.

The Laboratory operates two laser resource facilities. The MIT Laser
Biomedical Research Center (LBRC), a Biotechnology Resource Center of the
National Institutes of Health, develops basic scientific understanding, new
techniques and technology for advanced biomedical applications of lasers; core,
collaborative and outside research are conducted. The National Science
Foundation-supported MIT Laser Research Facility (LRF) provides resources for
core research programs in the physical sciences for 13 MIT Chemistry and
Physics faculty. Information about the equipment and facilities of the LRF and
the LBRC can be found in the Spectroscopy Laboratory Researcher's Guide.

RESEARCH HIGHLIGHTS

Professor Field and Dr. Steven Coy have developed a powerful pattern
recognition technique, extended spectral cross-correlation, to extract patterns
(relative intensities, energy splittings) repeated in two or more spectra
without any prior knowledge of the nature of the patterns or even the number of
repeated patterns present. Using this technique, complete information has been
extracted from the dispersed fluorescence (DF) spectrum of acetylene about the
early time (t < 1ps) dynamics of a "Franck-Condon bright state" on the
electronic ground state potential surface, at all energies up to 16,000
cm-1 above the zero-point level.

Professors Field and Steinfeld have initiated a spectroscopic study of the
isocynanogen NCNC molecule to observe the NCNC->NCCN isomerization process
in the DF and/or stimulated-emission-pumping spectrum. Our initial
spectroscopic goal is to record the electronic spectrum of NCNC in the
ultraviolet region (250nm), first by direct absorption and then by cavity
ringdown spectroscopy.

Professor Field and Dr. Steven Drucker are developing methods for studying
triplet electronic states of small, unsaturated hydrocarbon molecules (e.g.
acetylene). Triplets are long-lived energetic species that often play an
unsuspected role in photochemical processes. Our first complete
prepare-probe-detect apparatus for triplet spectroscopy has been designed, and
the experiments are in progress.

Professors Steinfeld and Field and their students are investigating the use of
advanced optical techniques for atmospheric monitoring. They have recently
demonstrated that backscattered light preserves the phase information necessary
for FM detection. Strong backscattered FM signals were observed from molecular
iodine at a vapor density of 109-1010 cm-1,
suggesting that FM-based remote sensing could be a sensitive and versatile
technique for measuring atmospheric trace gases. The addition of FM capability
to pulsed lasers used in atmospheric remote sensing promises a considerable
enhancement of sensitivity in comparison to the traditional differential
absorption lidar. By using a frequency modulated pulse, the absorber
distribution could be obtained directly in the FM-detected response from a
single pulse. Moreover, since the amplitude of the FM signals is approximately
proportional to the ratio of rf to linewidth, relatively sharp
molecular absorption lines (pressure broadened at 1 atm to 5 GHz, or 0.1
cm-1), may be readily distinguished from the near-continuous
background attenuation due to particulate scattering or molecular aggregates.
This technique can be extended into the ultraviolet and mid-infrared regions,
where key atmospheric trace gases such as methane, non-methane hydrocarbons,
and nitrogen oxides can be detected.

Professor Bawendi is using a picosecond laser and time-correlated photon
counting to study the electronic properties of semiconductor quantum dots and
heterostructures containing those dots. Data on dilute samples of dots have
been used to develop models of relaxation mechanisms and fine structure in the
electronic transitions. Time-resolved studies of the heterostructures (close
packed arrays of dots) have been used to study and understand energy transfer
between dots. Professor Bawendi has also developed a new apparatus to study
the spectroscopy of individual quantum dots. His group has found that the
linewidth are ultranarrow (<0.120meV), a result which has important
implications for the physics and applications of the materials. They have also
begun to study Stark effects of individual dots, which is important for any
device application that uses dots and electric fields to modulate light.

Professors Marc Kastner and Bawendi studied charge transport in close-packed
dot heterostructures . They used the picosecond apparatus to study the
temperature dependence of charge separation dynamics following
photoexcitation.

Professor Lippard and his associates have used Raman spectroscopy to
investigate the reactions of dioxygen with diiron (II) and dicopper (I)
complexes as models for metalloenzyme active sites. The O-O stretching
frequencies of the resulting peroxo-bridged dimetallic complexes were measured
and used to characterize the species present. Fluorescence resonance energy
transfer studies were also carried out to investigate the interactions of high
mobility group domain proteins with cisplatin-modified DNA containing pendant
fluorescent donor and acceptor molecules.

Professor George Benedek and Drs. Jayanti Pande and Manoharan have investigated
molecular changes in the protein crystallin and eye lens using Raman
spectroscopy. Oxidative stress, which leads to a variety of non-enzymatic
modifications in crystallins, is the major cause of cataract formation. The
intensity of S-S and S-H stretching modes in Raman spectra has shown that
sulfur centered oxidative dimerization occurs in crystallins

Professor Alexander Rich and Drs. Imre Berger, Dasari and Manoharan have
demonstrated the specificity of human double-stranded RNA deaminase enzyme for
left-handed Z-DNA using Raman spectroscopy. Raman spectra of B-DNA/Za-peptide
complex exhibit characteristic Z-DNA peaks at 627 and 1318 cm-1,
which are not observed in either B-DNA or Za-peptide. The occurrence of these
bands in the B-DNA/Za-Peptide complex shows that poly d(GC) DNA adopts a Z-DNA
conformation and binds to the Za-peptide. This study provides the first direct
evidence for the actual existence of left-handed DNA in the protein-DNA
complex.

Professor Ali Javan's research has focused on the resistance vs. voltage
characteristics (RVC) and optical response of superconductor-normal metal Point
Contacts (SNPCs). SNPCs are the simplest means of constructing nanoscale
conductance paths between a bulk super conductor and a metal. Novel RVC
features measured on these SNPCs have been shown to result from flow of
critical current in the SNPC. The optical response has been used in a new
technique to measure superconductor relaxation rates in real time.

Professor Kleppner and his students have extended our understanding of the
connections between quantum mechanics and classical motion by their new
technique of recurrence spectroscopy in a microwave field. Periodic orbits in
their system, a Rydberg atom in an electric field, can be identified from the
Fourier transform of the spectrum. They found that by applying microwave
fields near resonance with the periodic orbits, the intensity of the
recurrences was systematically modified in a fashion that could be related to
the detailed motion of the corresponding classical system. These results
illustrate one case in which quantum mechanics can describe detailed classical
motion.

Professor Feld and Drs. Dasari and Kyungwon An have studied the single-atom
microlaser, a fundamental laser device with a single atom as the gain medium.
Recent progresses include demonstration of a traveling-wave atom-cavity
interaction in the microlaser and development of a precision spectroscopic
technique to measure mirror absorption at sub-ppm levels using thermally
induced optical bistability.

Professors Tanaka and Feld and Drs. Kartha and Dasari studied spectroscopies of
gels near phase transitions. Random heteropolymers are known to be in three
phases: swollen, collapsed but fluctuating, and collapsed and frozen in a
conformation. The third phase, which is considered to be responsible for the
stability and memory of conformation of proteins, was found in copolymer gels,
where major interactions are hydrogen bonding. The degree of hydrogen bonding
is being studied using FT-IR and Raman spectroscopy.

Professor Feld and Drs. Dasari, Geurt Deinum, Irving Itzkan, Manoharan, and Lev
Perelman pursued basic and applied applications of lasers in biology and
medicine. Reflectance, fluorescence and near-IR Raman spectroscopy were used
for biochemical analysis of tissues and blood, and diagnosis of dysplasia,
cancer, atherosclerosis and other diseases. Clinical studies were pursued with
researchers from the Cleveland Clinic Foundation, Brigham and Women's Hospital,
Metrowest Hospital and New England Medical Center in colon, Barretts'
esophagus, bladder, breast and coronary and peripheral arteries. Quantitative
analysis of blood analytes using Raman spectroscopy is under development.
Observation of the nuclear signatures in reflectance spectra lead to a new
technique for measuring nuclear size distribution in biological tissues. UV
resonance Raman spectroscopy was used to characterize dysplasia. Photon
migration using a newly developed time-resolved optical tomographic system was
used to image small fluorescent objects (lesions) imbedded in turbid biological
tissue in the presence of background fluorescence, and to study paths of early
arriving photons. Finally, the mechanism of pulsed laser ablation of soft and
hard biological tissues was shown to be thermoelastic in origin. The
experimental and theoretical work being conducted in this program is advancing
new laser diagnostic technologies in the field of medicine.